Editing Altitude sickness (section)

=== Severe symptoms ===
Symptoms that may indicate life-threatening altitude sickness include:
; [[Pulmonary edema]] (fluid in the lungs)
: Symptoms similar to [[bronchitis]]
: Persistent dry cough
: Fever
: Shortness of breath even when resting

; [[Cerebral edema]] (swelling of the brain)
: Headache that does not respond to analgesics
: Unsteady gait
: Gradual loss of consciousness
: Increased nausea and vomiting
: Retinal hemorrhage

The most serious symptoms of altitude sickness arise from [[edema]] (fluid accumulation in the tissues of the body). At very high altitude, humans can get either [[high-altitude pulmonary edema]] (HAPE), or [[high-altitude cerebral edema]] (HACE). The physiological cause of altitude-induced edema is not conclusively established. It is currently believed, however, that HACE is caused by local vasodilation of cerebral blood vessels in response to [[hypoxia (medical)|hypoxia]], resulting in greater blood flow and, consequently, greater capillary pressures. On the other hand, HAPE may be due to general vasoconstriction in the pulmonary circulation (normally a response to regional ventilation-perfusion mismatches) which, with constant or increased cardiac output, also leads to increases in capillary pressures. For those with HACE, [[dexamethasone]] may provide temporary relief from symptoms in order to keep descending under their own power.{{citation needed|date=September 2022}}

HAPE can progress rapidly and is often fatal. Symptoms include fatigue, severe [[dyspnea]] at rest, and cough that is initially dry but may progress to produce pink, frothy [[sputum]]. Descent to lower altitudes alleviates the symptoms of HAPE.

HACE is a life-threatening condition that can lead to coma or death. Symptoms include headache, fatigue, visual impairment, bladder dysfunction, bowel dysfunction, loss of coordination, paralysis on one side of the body, and confusion. Descent to lower altitudes may save those affected by HACE.